The relationship of genetic factors, for example, the Avy gene of strain C3H-AvyfB, and non-genetic factors, for example, hormones, the various lines of mammay tumor virus (MTV), environment in mammary tumorigenesis in female mice continues. Interest is also in the way the various MTV's are transmitted: by female or by both sexes. Finally, the strains of mice that are capable of developing the interesting hormone dependent mammary gland lesions associated with pregnancy are also being investigated.